1. Field of the Invention
The present invention relates to a three-component measuring seismometer having an accelerometer that measures accelerations due to earthquake motion by using planar optical fiber rolls whose light propagation characteristics change sensitively in response to changes in pressure applied thereto.
The present invention also relates to a seismometer that detects accelerations in the directions of two orthogonal horizontal axes and in the vertical direction. The seismometer has load members displaceable relative to each other, and stacks of force sensors disposed in the gaps between the load members. Each force sensor has an optical fiber roll whose light propagation characteristics change in response to changes in pressure applied thereto. The optical fiber roll is provided on a rigid plate.
2. Description of the Related Art
Incidentally, the accelerometer is a sensor that outputs signals proportional to acceleration components of ground motion inputted thereto during earthquake(s). The accelerometer has a pendulum acting as a steady point in a container. Therefore, even if the container moves together with ground motion, the accelerometer can output the acceleration components of the relative motion between the container and the pendulum acting as a steady point. In general, the output signal from the accelerometer consists of a total of three components, including one vertical component and two horizontal components.
Conventional accelerometers include a mechanical type and an electromagnetic type. Of the two types, the electromagnetic accelerometer is more widely used. In this type of accelerometer, an electromagnet is placed around a pendulum wound with a coil of copper wire. Accordingly, acceleration components of ground motion obtained from the pendulum are detected as electric signals. A servo accelerometer, which is a kind of electromagnetic accelerometer, measures accelerations with high accuracy by virtue of the arrangement that a rebalance electric current proportional to the displacement of the pendulum position caused by an input ground motion is f.sub.e d back to the pendulum coil to hold the pendulum in the vicinity of the position assumed when no ground motion is applied. To measure minute accelerations with high accuracy using these accelerometers, it is necessary to place the accelerometer on an attitude controller so as to hold it as nearly horizontal as possible.
The conventional accelerometers measure the acceleration components of the relative motion between the seismometer container and the pendulum acting as a steady point and generally require a costly attitude controller to measure minute accelerations with the conventional accelerometers. Further, signal outputs from the conventional accelerometers are analog signals.
Incidentally, it is conjectured that optical signals will be used as data transmission means not only for seismic observations at the bottom of the sea but also for those on land in the future. In the case of on-line seabottom seismic observation systems in particular, optical cables are used; therefore, the conventional accelerometers execute signal processing for each channel through an analog-to-digital signal converter (A/D converter) and then convert the resulting signal into an optical signal through an electric-to-optical signal converter and send out the signal through an optical cable.
The observation systems using the conventional accelerometers require a sensor control mechanism which locks the pendulum of the seismometer during transport and installation of the equipment to protect the observation sensor from shocks and sets the pendulum free under preferable observation conditions. The sensor control mechanism needs an optical-to-electric signal converter and a sensor control mechanism unit comprising a motor, etc. because control commands from a land station are sent by optical signals.
Therefore, in the conventional seabottom seismic observation systems using the above-described accelerometers, for example, the observation sensors must be formed using components of fairly high quality and reliability which are on the same levels as those of components of submarine repeaters used in long-life submarine communication cables. In addition, manufacturing of mechanical components requires a considerable degree of machining accuracy and hence a fairly high degree of skill.
There is an increasing tendency to transmit land seismic observation data by digital-only lines using optical cables. Therefore, there is an increasing demand for observation systems that do not need components such as A/D converters and electric-to-optical signal converters. The currently existing observation systems incorporating the conventional accelerometers still have a large number of components.
In general, the smaller the number of components, the higher the reliability of the observation system. For the reasons stated above, seabottom observation systems using the conventional accelerometers inevitably require a great deal of cost for the production, transportation and installation.